1. Field of the Invention
The invention relates to an exhaust gas purifying apparatus for an internal combustion engine and a method for controlling the same apparatus.
2. Description of the Related Art
A known internal combustion engine includes a first exhaust passage and a second exhaust passage into which a common exhaust passage is bifurcated, NOx absorbent being provided in each of the first exhaust passage and the second exhaust passage, and the NOx absorbent storing NOx contained in exhaust gas when the air-fuel ratio of incoming exhaust gas is lean and releasing NOx stored therein when the air-fuel ratio of incoming exhaust gas is rich. In this internal combustion engine, in each of the first exhaust passage and the second exhaust passage, a fuel valve is provided upstream of the NOx absorbent and an exhaust control valve is provided downstream of the NOx absorbent (For example, see Japanese Patent Application Publication No. JP-A-7-102947).
In the internal combustion engine described above, when releasing NOx stored in the NOx absorbent in the first exhaust passage, the air-fuel ratio of exhaust gas in the first exhaust passage is made rich by closing the exhaust control valve in the first exhaust passage so as for the exhaust gas in the first exhaust passage to remain therein and then supplying fuel from the fuel valve in the first exhaust passage. Likewise, when releasing NOx stored in the NOx absorbent in the second exhaust passage, the air-fuel ratio of exhaust gas in the second exhaust passage is made rich by closing the exhaust control valve in the second exhaust passage so as for the exhaust gas in the second exhaust passage to remain therein and then supplying fuel from the fuel valve in the second exhaust passage.
However, in the above internal combustion engine, there is a problem that two fuel valves are necessary for releasing NOx from the NOx absorbent. More importantly, since the fuel valves are arranged quite far from the engine, a fuel supply pipe needs to be extended thereto, which is another problem.